1. Field of the Invention
The invention relates to a method for the mechanical cleaning of a powder, in particular of primary aluminum oxide, which is directed against a surface in order to strip particulate impurities adhering to its surface. The invention also relates to a device for the mechanical cleaning of a powder by mechanical detachment of particulate impurities from the surface of the powder, in particular of primary aluminum oxide, having a stripping device, within which the powder can be directed against a surface.
Such a method is suitable for removing impurities which adhere to the surface of powder particles. Such methods are employed, for example, in the cleaning of powder-form aluminum oxide, which is intended for the production of aluminum. During aluminum production, the powder-form primary aluminum oxide is first used to clean flue gas produced during the smelting electrolysis. In that case the flue gas is directed past the primary aluminum oxide, and the particles, in the form of fluoride, iron, phosphorus, carbon, silicon, vanadium and nickel, which are evolved from the smelting electrolysis, adhere to the surface of the powder-form aluminum oxide. Before the aluminum oxide is fed to the smelting electrolysis, the impurities in the form of iron, phosphorus, carbon, silicon, nickel and vanadium must be removed, since otherwise the process would be enriched with them. That would have a detrimental effect on the quality of the aluminum and the efficiency of the process. The fluorine which is required for carrying out the smelting electrolysis must be collected and fed back to the process.
A method for the removal of impurities from the surface of powder-form particles is disclosed by Published French Patent Application 2,499,057, which is identical to Norwegian Patent 147,791 and corresponds to U.S. Pat. No. 4,525,181. In that method, an air stream which is loaded with the powder to be cleaned is directed against an impact plate. The effect of the impact is that the impurities adhering to the surface of the powder are detached. The impurities and the powder are then separated from each other by screening. That method is very expensive to control and furthermore has an excessively low efficiency in the case of a throughput of several tons per hour.
French Patent 7,732,072 discloses a method for the stripping of particulate impurities from the surface of powders. In that method, the powder to be cleaned is introduced into two crossed air jets. At a crossover point of those jets, the powder particles strike one another. In that case the particles rub on one another and the impurities adhering to the surface are detached. Since the stripped impurities are lighter than the powder particles, they can be removed with the aid of an air current, while the heavier powder particles fall downwards. That method is very difficult when it is used in a current large-scale process, since it is not constructed for the cleaning of large quantities of particles.
German Published, Non-Prosecuted Patent Application DE 16 07 465 A1, corresponding to UK Patent Application GB 1 199 303, discloses an impact crusher for crushing hard and medium-hardness material. The impact crusher is formed of a housing with impact plates which are disposed opposite beater arms of a rotor. The housing furthermore has a delivery guide for feeding the material, as well as an output opening. The impact plate is disposed below the delivery guide and has a roof-shaped construction. The impact plate has a width from place to place which is greater than half a diameter of the rotor.
U.S. Pat. No. 4,361,290 describes a rotating beater mill with which material can be reduced to three different sizes and separated. The beater mill is provided with a rotor, the arms of which have plates for reducing material fastened to them.
Published European Patent Application 0 337 137, corresponding to U.S. Pat. Nos. 5,058,815 and 5,215,269, discloses a hammer mill for reducing the size of ore and similar materials. The device is bounded by a cylindrical housing. Impact plates and a rotor are disposed inside the housing and a plurality of hammers is fastened to the impact plates at a defined mutual separation and is used for reducing the size of the ore or for projecting the ore against the impact plates.